We use Amazon RDS for PostgreSQL because RDS and Amazon DynamoDB are two distinct database systems. DynamoDB is NoSQL DB whereas RDS is a relational database on the cloud. The pricing will mainly differ in the type of application you are using and your requirements. For some applications, both DynamoDB and RDS, can serve well, for some it might not. I do not think DynamoDB is cheaper. Right now we are helping Companies in Silicon Valley and in Southern California go SERVERLESS - drastically lowering costs if you are interested in hearing how we go about it.
Amazon RDS for PostgreSQL or Amazon DynamoDB? If you need to follow RDBS concepts choose Amazon RDS (PostgreSQL). This choice will guarantee a stable relational database with a great support for years from PostgreSQL Global Development Group. But, you need a great scalable NoSQL database. You can use Amazon DynamoDB. The most important thing about this is that you can use RESTful HTTP API to access data.